i think this idea of "karmic" debt (whether it is believed to be accrued or pay-as-you-go) has to be looked at further.
firstly one has to look at what karma is, in a spiritual sense - as opposed to what it is in a physical sense. because people seem to get them mixed together. firstly in the physical sense it can be look at as the ongoing consequences of action (or inaction, which is still a choice made and acted upon).
spiritual karma is not physical, neither is our souls (or whatever it is that you call the part of us which continues when the mortal life is done). therefore physical terms and definitions become both useless and confusing.
if one assumes that the soul grows and matures over it's many lifetimes - as one sees in nature with all living things - and the soul finds itself in a new and different form each life then it is clearly not the name, personal life experiences or physical attributes which are repeated. so why would anything such as wounds, physical disabilities or misfortune repeat? when it comes to karma in the spiritual sense, we must do away with any physical references.
so what is karma in the spiritual sense?
firstly we must place this firmly in the cycle of lives where it belongs - BETWEEN LIVES. not before, not after, not during but BETWEEN LIVES. and linked with karma, regardless of whether a person is a christian and only believes in one life, or a buddhist practicioner who believes in a cycle of lives, is judgement. judgement of the soul (and for the sake of clarity, by 'soul' i mean the part which continues after the mortal life is done). christians believe their souls are judged by some god, buddhists by their own selves. i tend to agree with buddhists on this point. our souls judge themselves.
but what do our souls judge themselves on? and what are the consequences of such judgement. if we accept that all living things have souls then judgement must be very broad indeed. for what can a sparrow or a lion do that it's soul would judge it for? we who call ourselves 'sentient life' are capable of 'good' and 'bad' to any degree. but animals, plants and fish, even insects and bacteria, are not. and given that in our broad span of lives, and that the proportion of life which might be considered 'sentient' is minuscule it can be said to be certain that nobody can claim that their souls never have been, and never will be, a non-sentient life.
and by reason this must be so. firstly, because to counter that a human soul is always a human soul and a rose soul is always a rose in it's lives callously denies the 99.99 percent recurring souls in the universe are capable of growing and experiencing life. that to me seems highly illogical. not only that but because of the sheer variety of life in the universe, to claim that all the universe's lessons can be learned in one life, or one form of life, seems to me to be foolish to believe as well.
so whatever the soul judges itself on, whatever determines how the soul grows and matures, all that can be understood in nature as to be reasonable has to be considered in any conclusion as to what you want to believe.